1. Field of the Invention
Methods and apparatuses consistent with the present invention relate to removing artifacts generated when a luminance signal and a chrominance signal are extracted from a composite video signal to improve picture quality and decoding the composite video signal.
2. Description of the Related Art
In general, a color TV video signal includes a luminance component and a chrominance component modulated within a restricted bandwidth. The luminance and chrominance components are separated from each other using a comb filter and decoded to be restored to the image corresponding to the color TV video signal. Artifacts caused by interference between the luminance and chrominance components can degrade the video signal depending on the performance of the comb filter. Accordingly, a variety of comb filtering techniques have been studied and developed for NTSC or PAL color decoding for past years in order to improve picture quality.
A conventional NTSC composite video signal encoder and decoder are will now be roughly explained.
FIG. 1 is a block diagram of the conventional NTSC composite video signal encoder. Referring to FIG. 1, R, G and B signals input to a color space converter 101 are converted into Y, I and Q signals. The Y signal passes through a low pass filter 102 having a passband of 0 through 4.2 MHz to be output as a luminance component. The I signal passes through a low pass filter 103 having a passband of 0 through 1.5 MHz and the Q signal passes through a low pass filter 104 having a passband of 0 through 0.5 MHz. Multipliers 105 and 107 respectively multiply the I and Q signals by phases having a difference of 90° between them to modulate the I and Q signals and output the I and Q signals to a summer 109. The summer 109 sums up the signals input thereto. The summed signal passes through a band pass filter 110 and is output to a summer 111 as a chrominance component. The summer 111 sums up the luminance component and the chrominance component to output a composite video signal CVBS.
FIG. 2 is a block diagram of the conventional NTSC composite video signal decoder. Referring to FIG. 2, the composite video signal decoder includes a comb filter 201 that can be used as a Y/C separation filter, a gate 202, a phase compensator 203, a voltage-controlled oscillator 204, a voltage source 205, a summer 206, a multiplier 207, a multiplier 208, a low pass filter 209, and a low pass filter 210.
The comb filter 201 filters a received composite video signal to output a luminance signal Y(t). The summer 206 subtracts the composite video signal from the signal filtered by the comb filter 201 and outputs the resultant signal to the multipliers 207 and 208. The multipliers 207 and 208 multiply the signals input thereto by a phase provided by the voltage source 105 and a phase having a difference of 90° from the phase to demodulate the input signals and output the demodulated signals to the low pass filters 209 and 210, respectively. Here, the voltage source 205 has a color sub-carrier frequency and the decoder demodulates chrominance signals using the color sub-carrier frequency. The low pass filter 209 passes the signal output from the multiplier 207 through a band of 0 through 1.5 MHz to output a chrominance signal I(t). The low pass filter 210 passes the signal output from the multiplier 208 through a band of 0 through 0.5 MHz to output a chrominance signal Q(t).
As described above, comb filtering separates the luminance signal and QAM-modulated chrominance signals from the composite video signal. However, crosstalk between the luminance signal and the chrominance signals generates an undesirable image artifact that is not included in the original signal when the composite video signal is transmitted using frequency interleaving. Thus, it is difficult to satisfactorily extract the luminance signal and the chrominance signals from the composite video signal. Though a technique of interleaving the frequencies of the luminance and chrominance signals restores the original signal using a comb filter theoretically, it is difficult to actually utilize the frequency interleaving technique because of spectral overlapping. Interference of the luminance and chrominance signals of the composite video signal, generated when the luminance and chrominance signals are extracted from the composite video signal, causes artifacts to appear in the image corresponding to the video signal. That is, there is a phase difference between two neighboring vertical lines of the chrominance signal in the NTSC video format. When an image is sampled along one vertical line, the phase difference between the vertical lines is changed and “dot crawl” pattern appears along the edge of a vertical line having color transition at the position where the phase difference is changed. This deteriorates picture quality.